A circuit arrangement of this type is described, for example, in the European publication EP 0 682 464 A1. This publication discloses a self-oscillating inverter having a starting circuit which is used to start the inverter oscillating. In addition, the circuit arrangement also has a device for deactivating the starting circuit. This device contains, as an important element, a transistor whose switching path, when switched on, forms a shunt around the charging capacitor of the starting circuit. After the inverter has begun oscillating, the transistor is switched on and the starting device is deactivated.
The European publication EP 0 753 987 A1 describes a circuit arrangement having an inverter to apply a medium or high-frequency supply voltage to one or more lamps and having a starting circuit which is used to start the inverter oscillation. In addition, this circuit arrangement also has a device for deactivating the starting circuit. This device comprises a resistor and a diode, via which the capacitor of the starting device is discharged after the inverter has begun to oscillate, so that the starting device is not able to produce any further triggering pulses to drive the inverter. In the case of a lamp which is defective or unwilling to fire, the inverter is stopped with the aid of a bistable shutdown device. In order to reset the bistable shutdown device and thus to permit the inverter to be restarted, the voltage supply to the inverter or to the lamp must be interrupted, at least briefly.
It is an object of the invention to provide a circuit arrangement for operating electric lamps which is improved as compared with the prior art.
According to the invention, this object is achieved by the features of patent claim 1. Particularly advantageous embodiments of the invention are described in the dependent claims.
The circuit arrangement according to the invention has an inverter for generating a medium or high-frequency supply voltage for one or more lamps and a starting circuit for the inverter, and also a device for deactivating the starting circuit, the starting circuit having a voltage-dependent switching means and a capacitor. The device for deactivating the starting circuit has a switching means whose switching path is arranged in parallel with the capacitor of the starting device. In order to control this switching means, the device for deactivating the starting circuit according to the invention is provided with a threshold switch. By means of this measure, deactivation of the starting circuit is ensured even when the inverter does not begin to oscillate. In addition, by means of the threshold switch, a time delay between the generation of the first starting pulse by the starting circuit and the deactivation of the starting circuit is made possible. As a result of the measure according to the invention, it is also possible for a simpler and more cost-effective shutdown device to be used, in order to shut down the inverter in the event of a defective lamp.
In addition, the threshold switch is advantageously arranged in such a way, and the device for deactivating the starting circuit is advantageously constructed in such a way that, after the supply voltage for the inverter has been switched on, the threshold switch is activated with a time delay with respect to the starting circuit. This ensures that the starting circuit is able to generate at least one or two trigger pulses in order to start the inverter oscillating before it is deactivated by means of the threshold switch.
A further advantage of the circuit arrangement according to the invention is that, instead of a bistable shutdown device, a shutdown device constructed as a threshold switch can be used, which is more cost-effective and has a simpler construction. Although the shutdown device constructed as a threshold switch is not bistable and, in particular, is not able to ensure a stable shutdown state without further measures, since the device for deactivating the starting circuit has a means which is used to maintain the deactivated state of the starting circuit when the supply voltage for the inverter is switched on and after the shutdown device has responded, and the device for deactivating the starting circuit interacts with the shutdown device constructed as a threshold switch, the circuit arrangement according to the invention ensures that after the shutdown device has responded, the oscillation of the inverter is terminated permanently, and it is possible to restart the oscillation of the inverter only after the supply voltage for the inverter or for the lamp has been switched on again.
In order to implement the aforementioned time-delayed deactivation of the starting circuit by means of the threshold switch belonging to the device for deactivating the starting circuit, the starting circuit and the device for deactivating the starting circuit advantageously have RC elements of different dimensions. The time constants of the two aforementioned RC elements are coordinated with each other in such a way that after the supply voltage for the inverter has been switched on, the threshold voltage of the trigger component of the starting device is reached earlier than the threshold device of the threshold switch belonging to the device for deactivating the starting circuit. For this purpose, the time constant of the RC element belonging to the starting circuit is advantageously smaller than the time constant of the RC element belonging to the device for deactivating the starting circuit.
In order to achieve a sufficiently short reset period for the device for deactivating the starting circuit, a discharge resistor is advantageously arranged in parallel with the RC element belonging to the device for deactivating the starting circuit, and is used to discharge the capacitor when the supply voltage for the inverter is switched off. The discharge resistor and the capacitor of the aforementioned RC element are dimensioned such that the product of the resistance of the resistor and the capacitance of the capacitor is less than 500 ms and preferably even less than 100 ms. As a result, the aforementioned capacitor is virtually completely discharged after the oscillation of the inverter has decayed.
As a central component, the device for deactivating the starting device has a switching means which is advantageously constructed as a transistor and whose switching path is arranged in parallel with the capacitor of the RC element belonging to the starting circuit, the threshold switch belonging to the device for deactivating the starting device being connected to the control electrode of the transistor and to the capacitor of the RC element belonging to the device for deactivating the starting circuit. In the low-resistance switching state, the switching path of this transistor forms a shunt to the capacitor of the starting device and therefore ensures that the capacitor belonging to the starting circuit is discharged or prevents this capacitor being recharged.
The means already mentioned above which, when the supply voltage for the inverter is switched on and after the shutdown device has responded, is used to maintain the deactivated state of the starting circuit, advantageously comprises an electrical connection between the voltage supply of the inverter and the control electrode of the transistor, the aforementioned electrical connection being routed via the threshold switch belonging to the device for deactivating the starting circuit. As a result, the transistor remains in the switched-on state after the oscillation of the inverter has been terminated by the shutdown device. The starting circuit therefore remains in the deactivated state.
In order to achieve the most immediate deactivation of the starting circuit, after the oscillation of the inverter has started, the control transformer of the self-oscillating oscillator (or an additional winding on a lamp inductor) is used, its primary winding being arranged in a load circuit of the inverter and its secondary winding being used to drive the control electrode of the transistor belonging to the device for deactivating the starting circuit. With the aid of the transformer, the start of the inverter oscillation is monitored and the aforementioned transistor is driven appropriately.